Generally, an optical card has extremely high information recording density as compared with other recording cards such as a magnetic card, and IC card and so on. The optical card has an optical recording section on one side of a substrate. Information is recorded onto the optical recording section by an optical energy, chiefly, a laser light. Based on the difference in optical reflectivity between a recorded portion and an unrecorded portion, the recorded information is optically read back.
The general background of the optical card constructed as described above is disclosed in U.S. Pat. No. 4,680,460 or U.S. Pat. No. 4,673,626, for example. As will be seen from these patents, the optical card is broadly divided into two categories. One of them is an read only optical card in which a user performs only reading-out, that is, an optical card of ROM (Read Only Memory) type. The other is an additional recording optical card in which the user performs also writing, that is, an optical card of WO (Write Once) type.
A first characteristic required for a recording material in the optical card of WO type is a writing characteristic. In view of this, a recording material of pitted type is preferable which is comprised of a low-melting-point metal such as tellurium, bismuth or the like or its alloy. Conversely, however, the more superior the writing characteristic, the more the recording material tends to be influenced by the outside environment, negatively affecting other characteristics such as durability and so on.
As a method of improving the environmental resistance, means are known for isolating the optical recording section made of pitted type recording material, from the outside environment. For example, an optical recording section of an optical disks is enclosed between a pair of substrates of the optical disc while providing an air gap. Here, the air gap is provided for recording material that is melted due to heat, in order to prevent the writing characteristic from being deteriorated.
A technique is also known in which the closing method having an air gap is applied to an optical card (Japanese Patent Provisional Publication No. SHO 61-96539). It cannot be said, however, that the technique is suitable for wide usage such as carrying or the like of the optical card, because it is believed that the air gap causes disadvantages such as reducing compactness and strength of the card.